Problem: In history class, the girl to boy ratio is $10$ to $8$. If there are $16$ boys, how many girls are there?
A ratio of $10$ girls to $8$ boys means that there is a group of $10$ girls for every group of $8$ boys. If there are $16$ boys, then there are $2$ groups of $8$ boys each. Because we know the ratio is $10:8$ , there must be $2$ groups of $10$ girls each. There is a total of $20$ girls in history class.